The present invention relates to the field of electric components and is intended to be used in the construction of gas-discharge overvoltage arresters which, in addition to a center electrode, have two end electrodes and are developed by means of an accessory device so that they have a so-called fail-safe behavior and possibly also a so-called vent-safe behavior.
One known overvoltage arrester of this type includes an annular center electrode, two hollow-cylindrical insulating bodies arranged laterally thereof, and two end electrodes arranged at the ends of the insulating members. A two-arm spring clip is attached to the center electrode, the ends of the two arms resting against the circumference of the two electrodes with the interpositioning of an insulating spacer. A coating of a polyurethane resin is employed as a spacer. This resin coating can melt upon overheating of the arrester, as a result of which the end of the arm of the spring clip comes into contact with the end electrode and thus short-circuits the overvoltage arrester. This property of the overvoltage arrester is referred to as "fail-safe behavior". Furthermore, the ends of the arms of the spring clip are developed with a sharp edge so that a high electric field strength may be developed on these sharp edges. This is of importance when the overvoltage arrester has failed due to a lack of Lightness in the housing. In that case, overvoltages which occur can be discharged subsidiarily via the air discharge gap formed between the sharp end of the arm of the spring clip and the end electrodes. This property of the overvoltage arrester is referred to as "vent-safe behavior" (see U.S. Pat. No. 4,912,592).
In order to produce fail-safe behavior in the case of a three-electrode overvoltage arrester, a construction is also known in which the ends of the arms of the spring clip rest axially rather than radially against the end electrodes, with the interposition of a fusible plastic member. Upon overheating of the arrester, the substantially cylindrical plastic member melts, as a result of which the spring clip comes against the end electrode and thus short-circuits the overvoltage arrester (see U.S. Pat. No. 4,984,125, FIG. 1a). In this known embodiment of an overvoltage arrester, both the center electrode and the end electrode are provided with radially extended connecting wires.
For overvoltage arresters having only two electrodes, it is furthermore known to obtain fail-safe behavior by the use of a disk of solder material of low melting point, a contact device which is under axial spring force being clamped against the disk of solder material and being displaced axially upon the melting of the solder disk, thereby short-circuiting the overvoltage arrester. In order to assure the vent-safe behavior, a back-up in the form of an air discharge gap is provided on the one electrode in the case of this known overvoltage arrester (see U.S. Pat. No. 4,366,412).
In the aforementioned three-electrode overvoltage arresters including a spring clip fastened to the center electrode, only a point contact between the spring clip and the end electrodes is formed in the event of thermal overloading of the arrester. In the event of very high short-circuit currents, there is therefore the danger that the ends of the arms of the spring clip will be thermally destroyed.